IceDock provides, window manager neutral (not yet there), support for WindowMaker, Afterstep(Wharf) docked applications. It was heavily influenced by Blackbox's Slit (both code and ideas stolen). Primarily aimed at IceWM.
IceDock is written in OCaml programming language, so you must have it installed if you plan to build IceDock from sources.
Download:
Problems: